Global Variable/Node across multiple materials?
This can be done with node groups.

I think you were trying to edit the attributes of the node group instance itself. Instead, try editing/animating the attributes of the nodes inside the group:



Just add a value node (or any input nodes you want, e.g. RGB for color) and press &#9096; CtrlG to make it into a group. 

Changes the values of the nodes inside the group will take effect everywhere the group is used, even across different materials. So in this case changing the value node will change the brightness of the diffuse material and the AO material.